Like a real Taser the 800kV will collapse under load. Once the skin is breached a human is suprisingly conductive. At 20cm apart a taser output voltage is approx 600v, not 50kV. It's not the voltage that hurts, it's a very specific waveform that interferes with motor control nerves.
The other issue you have to consider is that 800kV breaks over, give or take a bit, 40 inches in air. To put that simply the transformer terminals have to be 40 inches apart or the arc will start. You simply cannot have 800kV and a gap that short.
In general these devices, and I have looked at these transformers up close, are way, way less than 800kV, perhaps more like 15-25 kV.
I went back and looked at the video again, I'm guessing 20mm between pins, so at most 15kV. Once the arc initiates the impedance of the plasma is 70ish ohms for that length in air, so the transformer cannot sustain the arc. The arc either reforms once the transformer magnetics recover, or it waits for the next AC cycle.